Osmotic dehydration is relatively slow so acceleration of mass transfer would be advantageous. There are various methods to increase the mass transfer, such as application of ultrasound, high hydrostatic pressure, high electrical field pulses, vacuum and centrifugal force and microwave
Application of ultrasound
During osmotic dehydration Ultrasound in the food industry is relatively new and it has not been explored indepth until recently (De Gennaro et al., 1999). Ultrasound has been applied in the food industry to determine food properties due to low-frequency, high-energy ultrasound. It can travel through a solid medium; therefore, it can influence mass transfer.
A phenomenon known as acoustic cavitation is generated during the application as ultrasonic waves can generate minute vapor-filled bubbles that collapse rapidly or generate voids in liquids. Consequently, rapid pressure fluctuations are induced within the wet material by the ultrasonic waves. Ultrasound can be carried out at ambient temperature as no heating is required for reducing the potential of thermal degradation (Rodrigues and Fernandes, 2007).
It can influence mass transfer through structural changes, such as “sponge effect”, and microscopic channels (Carcel et al., 2007). Applying ultrasound dur- Azarpazhooh, Ramaswamy – Osmotic Dehydration 92 Drying of Foods, Vegetables and Fruits ing osmotic treatment has a significant effect on the kinetics of water loss, sugar gain, and firmness loss, as well as on the microstructure of osmotically dehydrated different products and processes in liquid–solid system, such as osmotic dehydration of apples (Carcel et al., 2007).
Application of blanching as a pretreatment
Hot water or steam blanching is a pretreatment before osmotic dehydration with the purpose of enzyme inactivation, and also to promote gas removal from surfaces and intercellular spaces; oxidation, discoloration, and off-flavor development and microbial growth are thereby prevented (Rahman and Perera, 1999).
Blanching has been applied prior to drying of fruits and vegetables, however blanching has some drawbacks such as causing changes in the chemical and physical state of nutrients and vitamins as well as having an adverse environmental impact on large water and energy usage (Rahman and Perera, 1999). Water blanching (85–100 °C) usually results in loss of nutrients such as minerals and vitamins